Photovoltaic devices are generally understood as photovoltaic cells or photovoltaic modules. Photovoltaic modules ordinarily comprise arrays of interconnected photovoltaic cells.
A method to manufacture photovoltaic devices and/or photovoltaic cells includes for example slicing of semiconductor material into wafers. Another method to manufacture photovoltaic devices includes deposition of semiconductor material as a thin film onto a substrate. The manufacture of thin-film photovoltaic devices may be more cost efficient than that of photovoltaic devices from wafers. Increased cost efficiency is achieved not only thanks to material and energy savings during production but also to technological progress in increasing the devices' photovoltaic conversion efficiency. The present disclosure relates to the manufacture of thin-film photovoltaic devices using a relatively low cost and low substrate temperature method, said devices having a photovoltaic efficiency that is higher than that of prior art thin-film devices manufactured at similar substrate temperature levels. Reductions in costs of photovoltaic devices for a given electrical power output are a major driver to expand their commercialization and help reduce emissions resulting from fossil fuel combustion. Furthermore, increases in photovoltaic device conversion efficiency enable higher electrical power output per unit area and therefore lower material and installation costs for a given output power.
A thin-film photovoltaic device is ordinarily manufactured by depositing material layers onto a substrate. From a simplified functional viewpoint, the material layers can be represented as a photovoltaic absorber layer possibly coated by a buffer layer, the combination being sandwiched between at least two conductive layers. The present disclosure is concerned with photovoltaic devices containing an absorber layer generally based on an ABC chalcogenide material, such as an ABC2 chalcopyrite material, wherein A represents elements in group 11 of the periodic table of chemical elements as defined by the International Union of Pure and Applied Chemistry including Cu or Ag, B represents elements in group 13 of the periodic table including In, Ga, or Al, and C represents elements in group 16 of the periodic table including S, Se, or Te. An example of an ABC2 material is the Cu(In,Ga)Se2 semiconductor also known as CIGS. The disclosure also concerns variations to the ordinary ternary ABC compositions, such as CuxInySez or CuxGaySez, in the form of quaternary, pentanary, or multinary materials such as Cux(In,Ga)y(Se,S)z, Cux(In,Al)ySez, Cux(Zn,Sn)ySez, Cux(Zn,Sn)y(Se,S)z, or (Ag,Cu)(In,Ga)ySez.
The disclosure presents a method for production at relatively low substrate temperatures (below 600° C.) of photovoltaic devices. It is especially advantageous for the production of flexible photovoltaic devices based on plastic substrates or metal foils. The disclosure also presents devices with a novel characteristic depth distribution of semiconductor elements in the absorber layer.
The photovoltaic absorber layer of thin-film ABC or ABC2 photovoltaic devices can be manufactured using a variety of methods such as vapor deposition, sputtering, printing, ion beam, or electroplating. The most common method is based on vapor deposition or co-evaporation within a vacuum chamber ordinarily using multiple material evaporation sources. U.S. Pat. No. 4,335,266 describes methods for forming thin-film solar cells from I-III-VI2 chalcopyrite compounds and is considered generally as a landmark in the art of manufacturing ABC2 photovoltaic devices. More recent prior art is presented in U.S. Pat. No. 5,441,897 which presents a method of fabricating Cu(In,Ga)(Se,S)2 thin-film solar cells in two or three steps. U.S. Pat. No. 6,258,620 contributes further to the aforementioned three steps method by using different material atomic ratios at the beginning of the deposition process and possibly contributing more material deposition steps to compose a precursor layer that is thereafter converted into an absorber layer by heating deposited materials at a substantially higher substrate temperature.
Although some prior art has enabled the fabrication of photovoltaic devices whose conversion efficiency may be on par with the more conventional technology of silicon wafers, high efficiencies of thin-films have so far been obtained using high temperature deposition processes, typically around 600° C. This disclosure therefore describes a method that has the advantage of also enabling the manufacture of high efficiency photovoltaic devices at substantially lower deposition temperatures, typically between 350° C. and 550° C. The disclosure also describes the characteristics of such photovoltaic devices.